1. Field of the Invention
The present invention relates to a thin steel band which is heat-resistant and corrosion-proof at high temperature and has excellent workability. More particularly, the invention relates to a low-cost thin steel band which comprises a thin steel sheet and a mutually dispersed layer formed on the surface of the thin steel sheet by a simple method and composed of the components of the steel sheet, nickel and aluminum, which is excellent in workability and heat-resistivity and is easy to fuse.
2. Description of the Related Art
Thin steel bands have been used in a variety of fields, and for the particular field in which thin steel bands must be heat-resistant and corrosion-proof at high temperature, satisfactory steel bands have not been available by this time.
This particular field is exemplified by exhaust pipes, mufflers or the like of an automobile, and more specifically a metal-made carrier body for an exhaust gas cleaning catalyst. Hereinafter, the characteristics required for thin steel bands will be described in connection with a metal-made carrier body.
Metal-made carrier bodies of the above sort, which are adapted to carry thereon an exhaust gas cleaning catalyst, include those having a structure formed by laminating at least one sheet-like metal band and at least one corrugated metal band in layers (laminated type) or rolling them together (rolled type) into a multi-layered composite body so as to increase the carrying area per unit volume, namely, so as to increase as much as possible the effective area of contact between exhaust gas and the exhaust gas cleaning catalyst per unit volume and further and also so as to reduce the own weight of the metal-made carrier body to a maximum possible extent. As it is in the form of a honeycomb, this structure is hereinafter called "honeycomb core structure".
For example, a sheet-like metal band made of a heat-resistant thin steel sheet having a thickness of 0.1 mm or small and containing 20% of chromium and 5% of aluminum, and a corrugated band made from another thin steel sheet of the same type are superposed one over the other to have areas of contact therebetween. They are then rolled together spirally into a honeycomb core structure defining many network-patterned gas flow passages along a central axis thereof for allowing exhaust gas to pass therethrough. The honeycomb core structure is enclosed within a tubular metal casing which has a single-layer structure and opens in opposite ends thereof. The constituent members of the honeycomb core structure, i.e., the sheet-like band and corrugated band are put together into a vibration-proof structure. Namely, the sheet-like band and the corrugated band as well as the honeycomb core structure thus rolled and the metal casing are put together at the areas of contact therebetween by welding, brazing or the like.
In the honeycomb core structure, the characteristics required for thin steel bands as its constituent members are to have adequate heat-resistance and corrosion-proofness at high temperature, since an exhaust system of an automobile is subject to high temperature of usually 800.degree. C. to 900.degree. C. and very corrosive exhaust gas.
For other characteristics, the thin steel sheet (starting sheet) must be easy to roll and must have remarkable rigidity so as not to allow the wavy shape of the corruguated band to be deformed during the production of the honeycomb core structure.
For the preferable thin steel sheet to be used as a constituent member of the honeycomb core structure, a heat-resistant stainless steel additionally containing cobalt or rare earth elements such as cerium, yttrium, etc. in order to improve oxdation proofness (U.S. Pat. Nos. 4,661,169 and 4,414,023) have been proposed. When using as a constituent member of the honeycomb core structure, a thin sheet of such know steel must have a thickness of 0.05 mm (50 .mu.m) and a width of 100 mm.
With the above required characteristics for the sheet-like and corrugated bands in view, an expensive heat-resistant steel such as a high-chromium steel, a high-nickel steel, etc. should be used; from a view point of cost, an inexpensive heat-resistant steel such as a low-carbon steel, a low-chromium steel, a low-nickel steel, etc. should be used.
However, if either the above-mentioned expensive heat-resistant steel or the above-mentioned inexpensive heat-resistant steel is used as the material for the constituent members, an adequately satisfactory honeycomb core structure cannot be obtained because of its poor workability and corrosion-resistance.
More specific problems are as follows:
i) If a heat-resistant steel containing chromium of 15% to 25% and aluminum of 2% to 5% is used, an adequately rigid corrugated band can be achieved, and especially there would be no deformation of the wave shape of the corrugated band during the production of the honeycomb core structure. Namely, when producing the laminated-type honeycomb core structure from a sheet-like steel band and a corrugated steel band, and also when inserting the honeycomb core structure in a tubular metal casing and fixing the honeycomb core structure on the tubular metal casing. However, this steel is very hard, and therefore a steel sheet is difficult to be rolled into a thin metal band for a honeycomb core structure. To obtain a steel band of a desired thickness, rolling and tempering must be repeated, which is laborious and time-consuming. Therefore it is expensive to manufacture a honeycomb core structure.
ii) In an attempt to obtain an inexpensive honeycomb core structure, it has been proposed to use a low-carbon steel containing less than 0.15% of carbon. A honeycomb core structure formed from the low-carbon steel is aluminized (forming solid solutions or alloying by dipping a honeycomb core structure in a molten aluminum liquid to cause mutual diffusion between the surface of the steel and the molten aluminum liquid). The resulting honeycomb core structure has no problem in corrosion-resistivity in an exhaust gas at a high temperature of 800.degree. to 900.degree. C. and also in cost of production. However, with this low-carbon steel, only a low-rigidity corrugated steel band can be obtained so that the wave shape of the corrugated steel band would be deformed markably when forming a honeycomb core structure.
Because of this markable deformation of the wave shape of the corrugated steel band, a desired height of the wave is difficult to achieve and therefore the mesh size of network-patterned gas flow passages in the honeycomb core structure would be reduced to cause disadvantages such as a pressure loss (lowering the efficiency of an internal combustion engine). Further, the contact between the sheet-like steel band and the corrugated steel band would change from spot contact to plane contact so that the amount to which an exhaust gas catalyst is to be carried on the carrier body can be reduced to impair the exhaust gas cleaning ability.
iii) Another attempt is to use an inexpensive heat-resistant steel such as a low-chromium steel, e.g. SUS410L (Cr content: 11 to 13.5%). This steel has a rigidity lower than the steel of i) above and higher than the steel of ii) above. Therefore, when forming a honeycomb core structure, the wave shape of the corrugated steel band would be deformed to a greater extent, compared with the steel of ii) above. The most significant problem of this steel is that it is inadequate in heat-resistivity and also in corrosion proofness. The same thing can be said when a low-nickel steel (nickel content: 3 to 6%) such as SUS201 and SUS202 are used as a heat-resistant steel.